Electrocoating with desulfated pigments

ABSTRACT

Pigments having sulfate impurity are treated with barium hydroxide. Such pigments are useful in producing glossy anodic deposits of pigmented polycarboxylic resins by electrocoating processes. The bath stability over long periods of time can be substantially improved and is valuable in continuous electrocoating processes.

United States Patent Schurr et al.

[4 1 Jan. 18, 1972 ELECTROCOATING WITH DESULFATED PIGMENTS Garmond G. Schurr, Palos Heights, lll.; Bruce M. Morgan, Griffith, Ind.

Inventors:

Assignee: The Sherwin-Williams Company, Cleveland, Ohio Filed: Sept. 22, 1969 Appl. No.: 860,023

US. Cl ..204/l81 Int. Cl. ..B01k 5/02, C23b 13/00 Field of Search .204/1 81 [56] References Cited UNITED STATES PATENTS 3,444,064 5/1969 Johnson v.204/181 Primary Examiner-Howard S. Williams Attorney-Richard G. Smith, Richard J. Lustig and Lowell G. Wise [57] ABSTRACT 3 Claims, No Drawings ELECTROCOATING WITH DESULFATED PIGMENTS BACKGROUND OF THE INVENTION This invention relates to treatment of inorganic pigments to remove impurities. In particular, it is concerned with the treatment of inorganic pigments to remove water-soluble sulfate impurity. Further, this invention teaches the use of bariumtreated pigments in depositing electrophoretic coatings on electrically conductive substrates.

There are numerous advantages of water-based eleetrocoating compositions over those employing organic solventslower cost of materials, uniformity of coating thickness, adaptability of electrocoating to continuous and easily mechanized paint lines, reduced toxicity and fire hazard, ability to deposit films on articles having complex configurations, and reduction of paint losses during application.

Water-based coatings have been used for many methods of application, including roller coating, brushing, dipping and spraying. The use of electrophoresis in depositing synthetic polycarboxylic resins from aqueous baths has emerged as an important industrial process. The most important economic uses of such electrocoating methods have been for painting base materials to provide a protective or decorative finish. In practice, electrocoating has been an immersion process wherein an organic polymer dispersed in an aqueous coating bath is transported toward a conductive surface which is biased with a direct current potential. The pigment materials of this invention have been found to have particularly good properties for electrocoating processes, and the examples given in describing the invention have been made primarilyfor this use.

After a pigment has been electrodeposited with a film-forming coating vehicle on a substrate from a liquid coating composition, a curing step is necessary to provide chemical resistance, hardness, etc. A curing process which has gained widespread acceptance in water based coatings is the crosspound, such as barium hydroxide, separating the treated pigment, washing the pigment, and recovering the desulfated pigment for use in an electrophoretic coating bath.

DESCRIPTION The present invention can be utilized in removing watersoluble sulfate impurities from pigrnentary solids. It is especially useful in treating titanium dioxide particles which are to be dispersed in an electrocoating bath. The source of sulfate impurity is not known to be significant in the treating process.

.The sulfate may be inherent in the raw material used for linking reaction of an aminoplast resin with I anotherobjectionable instability, it has been necessary in the past to 50 provide fresh mixtures of the pigment, aminoplast, cross-linking resin and hydroxy-containing prepolymer. This is disadvantageous in a production painting line, especially where the quality standards demand accurate blending of the components. The storage life of many prior art coating systems has been limited to a few days.

The primary visual effect of the unstable pigmented electrocoating is the drop in gloss of the cured film; however, the film texture also changes with aging. The ability to obtain a given film thickness in a standard deposition time was diminished considerably by the several days of aging some pigmented systems. This can be demonstrated in the larger voltage required to deposit a given amount of film during a 90- second period.

BRIEF SUMMARY OF THE INVENTION It has been found that the appearance of electrodeposited coatings from aged baths is affected by the presence of sulfate impurity in the aqueous bath. Furthermore, it has been found that this sulfate impurity may be introduced with pigment material used with the electrodeposition resin in the bath composition. This impurity can be removed by the methods of this invention, including a process of treating sulfate-containing pigments with a water-soluble barium compound.

manufacturing the pigment, or in the surface'treatment of the pigment. Sulfuric acid is used in many pigment manufacturing processes. For instance, the depositing of a silica or alumina surface onto a titanium dioxide core can be effected by acidifying an alkaline slurry with sulfuric acid.

When untreated sulfate-containing pigment is used to formulate electrocoating baths, deposits from freshly made baths do not have any significant decrease in gloss due to the pigmentation. However, after the bath has aged for several days, the gloss of a cured coating may drop more titan half its original value. The aging does not appear to have any significant efiect on unpigmented resin coatings or coatings containing sulfate-free pigments. There appears to be no correlation between the gloss decrease and pigment particle size.

The treating process comprises several steps: (1) contacting the sulfate-containing pigment in an aqueous slurry with an excess of water-soluble barium compound, (2) separating the treated pigment particles from the treating liquid, and (3) washing the treated particles to remove residual electrolyte. The treated pigments may be dried, or wet-mixed with the 'electrocoating composition. The pigment may be premixed with the electrodeposition resin prior to dispersing the mixture in a more diluted bath.

The contact time of the barium-treating process can be relatively short. Only a few minutes are required if a substantial stoichiometric excess of barium ion is used. Generally, about 15 minutes is satisfactory.

The relative ratios of pigment, water and water-soluble varium are not critical. Sufficient water should be used to permit agitation of the slurry. Large excesses of the barium compound permit shorter contact times.

The separation step can include centrifuging, filtering or both. The washing is preferably a sequence of small amounts of distilled water, although other pure volatile liquids may be used if they are solvents for the residual electrolytes. It is generally uneconomical to remove the electrolyte completely, and this method usually removes the total electrolyte slightly below the original level. Substantial amounts of nonsulfate electrolyte can remain with most pigments without having deleterious effect on the coating quality. In the examples, the washing purposely reduces the total electrolyte content to about the original level for comparative purposes.

A number of barium compounds are sufficiently water-soluble to be useful in treating the pigments for desulfation. The preferred barium compounds are the oxides and bases BaO, Ba(SI-I),-4 0, Ba(OH)z, and Ba(Ol-I),'8l-l,0. Barium salts of organic acids-acetic, propionic, butyric, oxalic-may be used. Also barium halides and nitrate are sufficiently water lieved to utilize the reaction of barium ion with sulfate ion to produce insoluble barium sulfate. When the barium treated pigment is introduced to the electrocoating bath, any sulfate present in the bath is in a water-insoluble fonn and cannot ionize.

The following examples are given to show the effect of 5 several commercial grade pigment materials on elec- TABLE 1 Bath Electrolyte Bath resist. Film Gloss Oil content Pigment No. pH (ohmem) (mils) Ago (1%) abs. (ohm-cm.) Composition 5'3 "328 1183 .5 Sil T10, min. 04%; A110. 3.8%; s102 0.0m.

132 18 .500 TiOrmin. 00%; A110; 2.2%; $10,029,. i'fi 14$ 16 A110: 3. Triethanolanrino {5g f, }1s.5 8,200 T 10,0n.w,11.0.3%. 7.2 11200 1.02 F 70 T101 mm 7.3 1100 1.07 1-w 67 24,000 A110: .07%-

1 25 14; 600 T10: min. 9 z a 5%; S100 0.3%; 2110 0.171. 0 13 7 300 7.2 1,100 1.00 1-w 00 1 7.5 1000 0.05 F 05 7 3 1'075 0.82 1-w 52 17 9-100 T101 A 10: 3%. 1891,.

5&3 10 8,700 T10,m1n.00%; 0.003%; s10 7.5 005 0.05 F 10 l 7.4 1 000 0,02 1 w 5 35 r %v 7 -0%,S10z0.6%,Zno 0.89;, 7.5 1',150 0.90 F 58 H 1.000 0.97 My 10 8,900 T1Ozm1n.94%Al1O;3.5%. 7.4 1,015 F 04 7.3 070 1-w 5o 1 Barium treated.

trodeposited films. Units are given in parts by weight unless otherwise specified.

The pigments tested included several commercially available titanium dioxide paint pigments having a particle size of less than 1 micron. These pigments consisted essentially of TiO with small percentages of surface treating materials (silica, zinc oxide, alumina, amine). The electrolyte content of these pigments was determined by the method of ASTM D2448-66T, in which the particles are admixed with water to dissolve the electrolyte and the resulting solution is measured in a Wheatstone bridge circuit to determine the electrical resistivity of the electrolyte. The resistivity (ohm-cm.) is inversely proportional to the electrolyte content.

EXAMPLE I a. A barium hydroxide solution is prepared by mixing 50 parts by weight of Ba(OH) -8H O with 950 parts of deionized water. To 800 parts titanium dioxide particles, having an electrolyte content equivalent to an electrical resistivity of 13,400 ohm-cm, and 200 parts water is added 400 parts of the barium hydroxide solution. This mixture is agitated for about 15 minutes and separated by filtering. The treated particles are washed twice with 600-800 parts of distilled water until the electrolyte content was reduced to the level of the untreated particles. The desulfated pigment particles were dried.

b. An electrocoating bath was formulated using the desulfated pigment of example I in an amount equivalent to a pigment volume concentration (PVC) of 12 percent in the cured film. The electrocoating resin comprised 80 parts by weight of an acrylic interpolymer having an acid value of 100 and 20 parts of an aminoplast cross-linking agent (hexaalkoxymethyl melamine). The acrylic polymer was 35 percent neutralized with alkali hydroxide base. Sufficient water was added to obtain a total solids content of 12 percent in the bath.

c. The electrophoretic deposition was conducted on aluminum panels in electrochemical cells having a circulating bath. The voltage was adjusted to produce a baked film approximately 1 mil (0.001 inch) in thickness during a 90- second deposition period. The deposited panels were rinsed with deionized water, baked for minutes at 350 F7, and the gloss of the film was measured using a standard 60 meter.

The procedure of example 1(b) and (c) was followed in electrodepositing a number of other pigmented films. The conditions were standard for all tests. The results of the experiments are shown in table 1, where the bath age is given as The treatment with barium according to the procedure of 40 ously during the l-week aging.

The barium treatment may be used for any sulfate-containing pigmented resin system where the water-soluble sulfate impurity poses a problem to the cured coating or bath stability. Although the examples given to describe the invention use acrylic interpolymers and aminoplast, it is within the inventive concept to apply the barium treatment to other waterdispersed systems, such as ester, epoxy, phenolic or other electrocoating resins.

The aging of the same acrylic electrocoating bath used in example I was also applied to pigmented systems containing no sulfate impurity and to unpigrnented resin baths. Using the acrylic-aminoplast system, an unpigrnented clear electrocoating bath was aged several weeks with no substantial change in the gloss of deposited coatings. Likewise, pigmented systems containing 3 percent PVC carbon black, and 6 percent PVC synthetic iron oxide showed no change in gloss after a oneweek aging.

What is claimed is:

2. The process of claim 1 wherein the pigment contains as its major component particles consisting essentially of titanium dioxide and wherein the pigment contains water-soluble sulfate as an impurity.

3. The process of claim 2 wherein the pigment is treated with a stoichiometric excess of barium hydroxide solution to precipitate the sulfate. 

2. The process of claim 1 wherein the pigment contains as its major component particles consisting essentially of titanium dioxide and wherein the pigment contains water-soluble sulfate as an impurity.
 3. The process of claim 2 wherein the pigment is treated with a stoichiometric excess of barium hydroxide solution to precipitate the sulfate. 